External configuration hardware of conventional gas turbine engines used to power aircraft and marine systems or used as industrial power generation sources generally comprises a plurality of tubes which provide fuel, oil and pressurized air to various engine components and subsystems. Due to generally restrictive installation volume routing requirements, the tubes are typically intricately convoluted, comprising a plurality of precise bends to provide proper clamping and end fitting locations. The materials utilized and processes employed to manufacture the hardware are selected to ensure a high degree of operational reliability. Further, tube contour and fitting orientation is tightly controlled as assembly stresses induced in the tube during installation due to improper contour can severely reduce tube life, oftentimes with dire consequences. For example, failure of a pressurized oil system tube during engine operation could result in loss of oil supply to the rotor bearings causing significant primary damage to the engine. Failure of a fuel system component, such as the main combustor fuel manifold tube, could result in degraded engine performance, flameout and possibly extensive secondary damage should a fire be initiated. All safety and performance critical tubes are therefore designed to meet rigorous operational requirements such as pressure, vibration and thermally induced stress cycling. Additionally, special care must be taken during manufacture, storage, transport and assembly to prevent nicks, kinks or other detrimental features which violate the integrity of the tube and may lead to premature failure.
An example of a particularly important system in a gas turbine engine is the main fuel distribution system. The system is designed for light weight, ease of maintenance and high reliability. By minimizing the number of separate components which must be brazed, welded or otherwise attached in a leakproof assembly, overall system reliability may be maximized. In a preferred system, two semicircular tubes comprise a main fuel manifold which circumscribes the engine proximate the combustor. The tubes are Joined together at the engine split lines with pressurized fuel being provided through a large inlet fitting. A plurality of equiangularly spaced T-fittings and short pigtail tubes are arranged around the manifold to provide fuel to respective fuel nozzles.
During manufacture, a straight section of tubing of appropriate diameter is bent in a forming die to create a smooth, continuous arcuate contour. A plurality of apertures are produced in appropriate locations, one per T-fitting, and the fittings are slid over the tube and brazed in place. To ensure high quality braze Joints and a reliable assembly, the gap between the tube and each fitting must be tightly controlled; therefore, the through hole in each fitting is of arcuate contour to match the arcuate contour of the manifold tube. Clearance for manufacturing tolerance, assembly and braze gap is nominally only one to three mils for one half inch diameter tubing. As can be readily appreciated, the straight tube sections must be of very uniform diameter and the bending process to form the arcuate contour must be tightly controlled to achieve a leakproof assembly. Local surface discontinuities such as ovalization, kinking, flattening or wrinkling of the tube prevent assembly of the fittings. Further, contour discontinuities, such as straight sections of tubing Joined by small radius bends, similarly prevent assembly.
Conventional manufacturing schemes rely on a rigid bending die having a constant radius of curvature die cavity face on an external circumference thereof. As is well known in the art, to produce a bend of a desired radius of curvature in an unrestrained tube, the tube must initially be bent to conform to a smaller radius of curvature to compensate for elastic springback of the tube material. The amount of springback in a tube varies depending on a plethora of geometric and metallurgical characteristics and oftentimes, while the die may produce an acceptable contour for a first tube, it may not for the next. Small variations in wall thickness or hardness due to minor differences in heat treat, while producing generally acceptable tubing which meets industry specification requirements, cause unacceptable fallout during tube forming. Tubes which fail to meet the contour requirement, for example a sixty rail volume envelope for a one half inch tube bent in a semicircle having a nominal radius of fifteen inches, must be manually reworked. Tubes which cannot be reworked to meet the volume contour requirement or suffer ovalization or other distress during manual adjustment cannot be utilized and are scrapped.
Prior attempts to solve tube forming variability in a systematic manner have proven to be cost prohibitive or of limited benefit. For example, instituting unique, highly restrictive tubing material processing and geometry specifications would be very costly to develop and implement. Alternatively, significantly relaxing the contour tolerance requirement would result in premature failure of tubing with excessive installed assembly stresses. Another alternative, producing a series of incrementally sized bending dies for each diameter, radius of curvature and material tube is costly, as well and an unacceptable option in a production environment. An adjustable die employing an expander concept with a plurality of radially adjustable wedge segments would produce unacceptable, nonuniform bends as discussed hereinbefore.